A bus, or computer universal switch, is essential for data transfer within a computer or between more than one. We'll take a look at topics like PCI, SCSI, USB Ports and serial ports.

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Please copy/paste the following text to properly cite this How Stuff Works article:

Wilson, Tracy V., and Jeff Tyson. "How SCSI Works." 18 May 2001. HowStuffWorks.com. <http://computer.howstuffworks.com/scsi.htm> 13 May 2008.

by Tracy V. Wilson and Jeff Tyson

Inside This Article

Introduction to How SCSI Works

SCSI Basics

SCSI Types

Controllers, Devices and Cables

Termination

Lots More Information

See all Buses articles

Controllers, Devices and Cables

A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus.

Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.

Internal SCSI devices connect to a ribbon cable.

Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device.

External SCSI devices connect using thick, round cables.

The cable itself typically consists of three layers:

Inner layer: The most protected layer, this contains the actual data being sent.
Media layer: Contains the wires that send control commands to the device.
Outer layer: Includes wires that carry parity information, which ensures that the data is correct.

Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors.